We finally managed to track down the cause of this rare error message.  

The error can only occur if you start ProSelect with your Preferences set to having the Default Number of Sets: None.

In this case, you will get the message:

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An Error occurred while saving the album

The Error message was:
Unknown Save Album Error (6)

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when you try to save a newly created album OR an album that was previously saved using version 4.08 or earlier with "No Sets" selected.

(Loading an album which was saved with one or more sets does not cause the error.)

Very early versions of ProSelect defaulted to the "No Sets" preference (more recent versions default to 3 sets) so, if you have upgraded from an earlier version, ProSelect would have picked up the earlier "No Sets" preference setting and carried this through to the newer version causing the problem.

Solution for current versions

The simple solution is to change your Preferences (as shown above) to

Default Number of Sets : 3  and then restart ProSelect.

Permanent Solution

We have updated ProSelect 2011r1 so that the Default Number of Sets: None  setting is no longer available so you can't accidently select this option.